1. Field of the Invention
The present invention relates to printing apparatus such as back printers or line printers and more particularly to a printing apparatus of the type which allows an operator to visually inspect the printed or typed information when the operator desires to do so or under predetermined conditions.
2. Description of the Prior Art
During use of a keyboard-operated printing apparatus having a type carrier which is arranged to travel along a horizontal print line, it is often desired by the operator to visually inspect and proofread the typed line or lines with the printing paper held in the printing position so as to see if the information that has been keyed in is correctly printed on the printing paper and the typed line or lines are properly located on the paper. If, in this instance, there exists an obstacle such as a portion of any of the type carrier, platen and inking ribbon between the operator's eyes and the line or lines on the paper which the operator desires to inspect, the line of sight of the operator is intercepted by such an obstacle so that the operator is unable to directly view the typed information without partially drawing out the printing paper from the printing position.
A printing apparatus has therefore been proposed which allows an operator to visually inspect typed characters when the operator interrupts entry of information at the keyboard. A representative example of the printing apparatus of such a nature is a back printer disclosed in U.S. Pat. No. 3,752,069. The back printer therein shown features, inter alia, a flexible type carrying band which is formed with spaced apart upstanding fingers each bearing a character to be typed. The character bearing fingers are backed up by a platen during printing and, each time the operator interrupts keying operation at the keyboard, the platen is moved out of its operative position behind the fingers and enables the operator to view the typed characters through the spaces between the successive fingers of the type carrying band as the band is driven to travel along a horizontal print line. One of the drawbacks encountered in a printing apparatus of this nature is that the line of the characters printed on the printing paper is obscured by the type bearing fingers which pass in succession over the front face of the printing paper so that the operator is permitted to recognize the printed information only by recourse to afterimages of the characters viewed and is accordingly unable to clearly and uninterruptedly proofread the typed messages. Another drawback is that the movable platen arrangement of the described prior-art apparatus owes its benefits to the type carrying band having spaced apart finger portions and is therefore not compatible in construction and in effect with ordinary type carrying bands, chains and trains which are void of such fingers. If, furthermore, an inking ribbon is used with a movable platen of the above described nature for applying ink to the printing paper, it is required to have the inking ribbon arranged to be withdrawn from its operative position together with the platen when the platen is moved out of the operative position thereof. The inking ribbon is disposed in such a manner as to form a small gap between the ribbon and the platen with the type carrying band movably passed through the gap so that not only intricate mechanical arrangements are indispensable for effecting simultaneous movement of the inking ribbon and the platen out of the respective operative positions thereof while allowing the type carrying band to freely travel through the gap but the inking ribbon tends to produce puckers when the ribbon is moved out of and back into the operative position thereof. Such puckers may hinder the printing paper and type carrying band from being fed and moved smoothly during printing operation. The platen incorporated in a printing apparatus using a horizontally travelling type carrying element is, still furthermore, preferably so arranged as to slightly protrude beyond the path of the type carrying element toward the print line in the apparatus for constantly producing a tension in the type carrying element so that the type carrying element is held in sliding contact with the rear face of the platen. Such an arrangement is broadly put to use for the purpose of enhancing the quality of the prints to be obtained and extending the lifetime of type carrying elements. When the platen in a printing apparatus of this nature is moved in a direction perpendicular to the direction of travel of the type carrying element while maintaining its sliding contact with the type carrying element, the type carrying element tends to be dragged in the particular direction due to the friction between the platen and the type carrying element. This not only hinders the platen from being smoothly moved out of and into the operative position thereof but gives rise to reduction in the service life of the type carrying element due to the added attrition between the platen and the type carrying element. The present invention contemplates elimination of all these drawbacks inherent in prior-art printing apparatus of the described characters.